The present invention relates generally to devices for transmitting electromagnetic signals of a desired frequency between a source and a load and more particularly to devices for transmitting electromagnetic signals of a desired frequency between a source and a load that additionally provide over-voltage protection to the transmission line.
A radio frequency (RF) transmission line is a structure that is designed to efficiently transmit high frequency radio frequency (RF) signals between a source and a load. An RF transmission line typically comprises two conductors, such as a pair of metal wires, that are separated by an insulating material with dielectric properties, such as a polymer or air. One type of an RF transmission line which is well known in the art is a coaxial electric device.
Coaxial electric devices, such as coaxial cables, coaxial connectors and coaxial switches, are well known in the art and are widely used to transmit electromagnetic signals over 10 MHz with minimum loss and little or no distortion. As a result, coaxial electric devices are commonly used to transmit and receive signals used in broadcast, military, police, fire, security and civilian transceiver applications as well as numerous other uses.
A coaxial electric device typically comprises an inner signal conductor which serves to transmit the desired communication signal. The inner signal conductor is separated from an outer conductor by an insulating material, or dielectric material, the outer conductor serving as the return path, or ground, for the communication signal. Such an electric device is typically referred to as coaxial because the inner and outer conductors share a common longitudinal axis. It should be noted that the relationship of the geometry of the conductors and the properties of the dielectric materials disposed between the conductors substantially define the characteristic impedance of the coaxial device.
It has been found that, on occasion, potentially harmful voltages are transmitted through RF transmission lines. In particular, radios operating in either the lower end of the ultra high frequency (UHF) band or lower frequency bands (i.e., below 500 MHz) often utilize longer antenna lengths to enhance performance compared to antennae used in higher frequency applications. In addition, the long range signal propagation characteristics of these lower frequencies allow for superior long range communication. Furthermore, since the mounting height of a radio antenna serves to increase its range, radio antennae are commonly mounted from an elevated position (e.g., a tower or mast). As a result, it has been found that radio antennae are highly susceptible to lightening strikes, the high electrical energy of a lightning strike increasing the likelihood of significant damage to any sensitive components connected to the transmission line, which is highly undesirable.
As a result, at least one RF transmission line component is commonly provided with protective means for deflecting undesirable electromagnetic impulses away from a load connected thereto. For example, it is well known in the art for a coaxial electric device to include a shunt conductor which connects the inner conductor either to the outer conductor or directly to ground. The operational frequency of protective devices which utilize shunt conductors is typically greater than 400 MHz because lower frequencies require excessively long shunt conductors. As can be appreciated, the use of excessively long shunt conductors is disfavored, among other reasons, for substantially increasing the overall size of the protective device. An example of a protective device provided with a shunt conductor for grounding undesirable impulses is shown in U.S. Pat. No. 7,440,253 to George M. Kauffman, which is hereby incorporated by reference.
Although well known in the art, coaxial electric devices of the type as described above typically suffer from at least some of following shortcomings.
As a first shortcoming, coaxial electric devices of the type as described above typically include an inner conductor that is assembled from multiple, individually machined pieces. Specifically, the inner conductor often includes a shortened, center pin that is externally threaded along its length and a pair of opposing end pins, each end pin comprising an internal threading at one end and a male or female connector at its opposite end. Accordingly, as part of the assembly process, the internally threaded end of each end pin is screwed onto a corresponding end of the center pin until the pair of end pins are drawn into conductive contact with one another. In this manner, a unitary center conductor is formed that includes either a male or female connector at each end. It is to be understood that if the device is provided with a shunt conductor (or other similar signal diverting element), a portion of the shunt conductor is typically wedged, or sandwiched, firmly between the end pins as they are drawn together on the center pin during the assembly process, thereby conductively connecting the shunt conductor to the inner conductor. As can be appreciated, it has been found that the utilization of a center conductor of the type as described in detail above significantly increases manufacturing costs. In particular, in order to provide each pin of the inner conductor with its associated threading, a complex machining process is required. Furthermore, the process of assembling the various pieces of the center conductor together and, in turn, to the shunt conductor necessitates a considerable labor requirement, thereby significantly increasing manufacturing costs, which is highly undesirable.
As a second drawback, coaxial electric devices of the type as described above include an outer conductor that is typically constructed entirely out of a highly conductive, hardened metallic material, such as brass, copper or the like, for performance purposes. However, as can be appreciated, the aforementioned materials that are traditionally used to form the outer conductor a coaxial electric device are relatively expensive in nature, which is highly undesirable. Furthermore, it is to be understood that if the outer conductor of a protective device were manufactured using a softer, less expensive conductive material, such as aluminum, the performance of the device may be compromised. Specifically, the inherent softness of alternative metals will ultimately result in their deformation in the region of contact during the coupling process. Accordingly, over time, this deformation of the material in its region of contact may result in insufficient conductive coupling, which is highly undesirable.
As a third drawback, in order to provide a conventional coaxial electric device of the type described in detail above with wideband capabilities, substantial modification of the configuration of the inner and/or outer conductor is typically required, which is highly undesirable in certain applications. In addition, it has been found that modifying the configuration of either the inner conductor or the outer conductor can in turn compromise the radio frequency (RF) performance of the device.